Current federal law prohibits the carrying of passengers for hire in single engine propeller-driven fixed wing aircraft. Therefore, propeller-driven commercial passenger airplanes have multiple engines, each driving a propeller. Such aircraft are expensive to purchase, operate, and maintain, especially for the small commercial and charter airlines that typically own such airplanes. Apparently, single engine propeller-driven airplanes are not approved for commercial passenger use because of their perceived unreliability.
Typically, twin engine propeller-driven airplanes mount an engine on each wing because engine configurations offer no reasonable alternative. However, wing-mounted engines reduce aerodynamic efficiency. They also require complex structures and expensive duplicate installations of components and systems. Wing-mounted engines also produce dangerous controllability problems resulting from sudden assymetrical thrust when an engine fails on a heavily loaded airplane.
For many years, helicopters have been permitted by federal law to transport passengers for hire so long as they are equipped with two engines to drive the main rotor shaft. Accordingly, it is believed that a single propeller-driven fixed wing aircraft having two engines available to drive the single propeller shaft should be acceptable under federal law for carrying commercial passengers if such a drive system is reliable and capable of operating on one of two engines if the other fails. Such a drive system would be especially valuable to small charter and commercial airlines if it were adaptable to existing single engine fixed wing aircraft and to existing engines, both turbine and piston driven.
As mentioned above, multiple engine drives for a single helicopter rotor are known. For example, U.S. Pat. No. 4,177,693 to Ivanko discloses the use of three turbine engines to drive the primary power shaft for a main helicopter rotor. The drive from each engine is into a common gearbox but with separate gearbox sections. The input shaft (62) into each input section has splines (64) which drive a gear (66) which drives an idler gear (70) through overrunning clutch (68). Idler gear (70) drives an output gear (78) on a shaft (80) splined to the main power shaft (18). Because the clutch is positioned between the input shaft (62) and the idler gear (70), any failure in any one of the three power trains downstream of its clutch could disable the main power shaft.
Dual engine drives for the main rotor of a helicopter are also known and include the Models PW-209T and PT-6T manufactured by Pratt and Whitney and the Model SA 365 manufactured by Dauphin. However, also such helicopter drive systems are believed to use overrunning clutches in their dual drive trains upstream of the final drive shaft. Therefore such drive systems have the same disadvantages as the Ivanko system.
In addition, multiple engine helicopter rotor shaft drives are not adaptable to fixed wing aircraft, and especially to existing single propeller-driven fixed wing aircraft because of the weight, center of gravity, and configuration constraints of such aircraft and engines suitable for use in such aircraft. For example, typically the multiple-engine helicopter drives described require a torque-combining gearbox directly connected to the engines. Therefore any abnormal torque or shock loading of the rotor is transmitted back to the engines, exposing them to potential damage, especially if they are turbine engines. Also, because of the configuration of a helicopter, the output shaft from the torque-combining gearbox must be coupled to the rotor shaft through a right angle gearbox, further complicating the drive system. However, probably the most serious drawback of such helicopter drive systems is the possibility of rotor malfunction in the event of a failure in any one of the multiple drive trains if that failure occurs downstream of the clutch in such drive train.
The use of multiple engines to drive a common propeller shaft in fixed wing aircraft has also been suggested, for example, as disclosed in U.S. Pat. No. 2,396,745 to Nallinger, et al., and U.S. Pat. No. 3,340,748 to Young. In Nallinger, et al., two engines drive a single propeller shaft through a common gear train. There is a clutch between the output shaft from each engine and the gearbox input pinion shaft from each engine.
In the Young patent, dual aircraft engines (17)(19) drive a single propeller shaft (21) through connection of the engine output shafts to a common gearbox (24) by belt-and-pulley drives. One or both engines drive the single propeller shaft, but the drive is at a different ratio when only one engine is used. The gearbox (FIGS. 5 and 6) includes complex planetary drives in which there are overrunning clutches (74)(71) between the primary input drive sleeves (40) and the primary input drive gears (65) and also between a secondary drive gear (61) and the main gear housing. There is no clutch between the planetary drives and the single propeller shaft (21) or its drive spindles (50).
The Nallinger and Young drive systems have the same drawbacks as the described helicopter drive systems. Any failure in one of the dual drive trains downstream of the clutch in such drive train is likely to disable the propeller shaft.
Dual engine drives for a single output drive shaft have also been suggested for other applications. For example, U.S. Pat. No. 3,669,230 to Burkhardt, et al., discloses a dual engine drive system for a single ship's propeller shaft. Dual engines (1)(1') drive a common drive or propeller shaft (3c) through reduction gearing, including a common output gear (3h) on the propeller shaft and separate pinions (3k) on each input shaft from the engines to the gearbox. A shiftable friction clutch (2) connects each engine output shaft to its gearbox input shaft. However, the Burkhardt system appears to be typical of dual engine drive systems for single output shafts in that the clutching is between the engine output shaft and its gearbox input shaft. Thus, any malfunction in the drive train from one engine downstream of the clutch will disable the final output drive shaft.
U.S. Pat. No. 4,106,603 to Walter discloses a dual engine drive system for a single propeller shaft in which the dual engines (1)(1') drive shafts (7)(7') through flexible couplings (2)(2'). The input shafts (7)(7') extend through hollow shafts (4d)(4'd) and are coupled to such hollow shafts through clutches (3)(3'). The hollow shafts carry gears (4b)(4'b) which drive a common output gear (4a) on the common propeller shaft (5). Thus, the Walter drive system has the same defect as the aforementioned drive systems in that any malfunction in the gear train of one engine downstream of its clutch, such as the gear teeth or gear shaft bearings, is likely to disable the entire drive system, including the propeller shaft.
Known dual engine drive systems for a single output shaft would be unsuitable for existing single propeller-driven fixed wing aircraft, for other reasons as well. Most lack other fail-safe features that would be required to ensure the reliability demanded for such aircraft. For example, the lubrication systems of known multiple engine drive systems are either not adaptable to fixed wing, single propeller-driven aircraft, or lack the built-in redundancies that would be required to ensure safe operation. Although the Ivanko patent suggests the use of independent oil systems for the separate gearbox power sections and the main drive section, the oil system of one section appears to be incapable of taking over for that in another section, if the system in the other section should fail. Furthermore, no suitable means exist for mounting a multiple engine-single propeller-driven system in a fixed wing aircraft. Although U.S. Pat. No. 4,531,694 to Soloy discloses a mounting system for mounting a single engine and separate gearbox as a unit in a single-engine fixed wing aircraft, it is not adapted for multiple engine-single gearbox-single propeller applications.
Aircraft and aircraft engines come in various sizes and configurations. Therefore, with known such drive systems it would also be necessary to design a different gearbox for each different configuration of aircraft and each different configuration of aircraft engine, a prodigious task.
Accordingly, there is a need for a multiple engine drive system for a single propeller shaft suitable for fixed wing aircraft use and for a gearbox and a mounting system for such a drive system.